Block Windows

A safe efficient option

By James Dulley

Glass- and plastic-block windows have few air leaks, protect against storm damage and provide security against break-ins.

As people become more concerned about home security, they are replacing more of their old, inefficient windows with glass- and plastic-block windows. This is particularly true for first-floor and basement windows where a would-be thief can hide and take time to quietly pry open or break a standard window.

Protection from severe weather is now also becoming a more widespread concern. Some standard pane-glass windows can withstand the force of the wind during a violent storm, but damage from flying debris often does much of the damage. Hurricane-resistant block windows can prevent this damage.

It is possible for an intruder to break through a glass-block window, but it would be very difficult, take quite a bit of time, and create a lot of noise.

Plastic-block windows are also available and look identical to true glass blocks. Most are molded from acrylic plastic, which is fairly impact-resistant, much more than standard double-pane glass windows, and it does not yellow over time.

Glass- and plastic-block windows can be energy efficient for several reasons. There is a sealed insulating air gap inside of each block. This works to be particularly efficient in glass blocks because the two halves are fused together under heat. When the glass blocks and the air inside the sealed gap cool, a slight insulating vacuum is created inside the block.

Just as most replacement windows now use low-E (low-emissivity) coating on the glass, so do glass and plastic blocks. Some Hy-Lite acrylic plastic blocks have an efficient low-emissivity coating on a third pane inside the block, providing an R-3 insulating value. This can be combined with a tinted block for summer heat rejection of SHGC (solar heat gain coefficient) = 0.27, plus winter heat savings.

Another plus: block windows are very airtight and remain so. There is very little, if any, air infiltration when the blocks are assembled properly in mortar or clear silicone. (When installing a completed block panel, make sure to caulk well around the frame perimeter.)

If you want the option of natural ventilation, several of the blocks can be replaced with a small hopper window. The opening panel is made from tough polycarbonate plastic. It is too small to get through even if someone could break it. A hopper-style window closes on compression-type weatherstripping, so it is very airtight.

Opening casement-style block window panels with privacy style blocks are often used in bathrooms and basements. Always check your local building codes about egress (escape) requirements for rooms in your house. If there is a fire or a roof collapses during a tornado or hurricane, you need to have a window that will open wide enough to crawl through. Many casement style block replacement windows should meet these requirements.

There is quite a learning curve to install individual blocks yourself, so if you are inexperienced, select preassembled panels. These large complete panels are installed similarly to any replacement window. Some of the strongest glass block panels, which meet IBC (International Building Code) and Dade County, Fla., hurricane impact tests, are framed by 2-by-6 pressure-treated lumber.

Glass-block panels, such as from Pittsburgh Corning, are available in 60 sizes and three block patterns. Plastic blocks, such as from Hy-Lite, can be custom-sized to fit your existing window opening.

About the Author

James Dulley is an engineer and syndicated columnist for the National Rural Electric Cooperative Association. www.dulley.com

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Energy Efficiency Tip

Adding insulation to your home? An R-value indicates insulation’s resistance to heat flow—a higher R-value means more effective insulation. Every type of insulation has a unique R-value depending on material, thickness, and density. Your ideal R-value depends on whether your home is new or existing, your heating fuel, and where you live.